bubbling mx5

[mongoose]

H all.For a little while now the wifes mx5(mk1 1.8) has been loosing coolant.Sometimes,when you park the car up,you can see the coolant in the expansion tank full to the top and bubbling due to air/water being forced into the tank from the rad overflow pipe.Pretty quickly this leads to a significant coolant loss through the overflow from the expansion tank,as you can imagine.Ive changed the thermostat to no avail.Theres no water in the oil or vice versa,any ideas please?cheers,Mke.

[grahambell]

Might be a cracked head.

You can sometimes get hairline cracks around the valve seats that won't let oil and water mix but will let the engine pressurise the cooling system when hot.

More likely with a cast iron head than an alloy one with valve seat inserts, but fits the symptoms.

[Mutant Rat]

Get a chemical test done on the coolant - just because there is no oil/water contamination doesn't mean you don't have a head gasket failure.

Chemical test will tell you whether there are combustion gases dissolved in the coolant. Cost about a tenner last time I had one done (a few years ago, so maybe £15 now), or buy a test kit yourself (about £35 IIRC).

Also might be worth bleeding the cooling system, as airlocks will sometimes cause these sort of symptoms (airlock causes localised boiling, which will cause bubbling in the expansion tank/radiator and will force coolant out through the pressure cap, leading to the coolant loss). If coolant is being lost in this way, you can often tell by staining around the pressure cap. I'm not familiar with the MX5 cooling system, though, so perhaps someone else could say whether airlocks are a problem and how to go about bleeding the system?

edited to add...the chemical test should identify cracked head or block that Graham mentions, too.

>> Edited by Mutant Rat on Friday 2nd September 19:31

[PJLarge]

It'll be a blocked radiator. MX5 rads are pretty well known for blocking up, it's made worse by the fact the rad has such a small capacity. Don't bother with the rad flushes you can buy, they don't work. A new rad is the order of the day!

Phil.

[mongoose]

Thanks for the replys folks.I prefer the sound of the blocked rad compared to the other alternatives!Could it still be blocked if my cooling has been fine though?-temp gauge is as its always been.

[PJLarge]

HGF is highly unusual on an MX5, as are cracked heads - it's an alloy head against an iron block. Your best bet will be to get the car up to temp on the drive, let the t-stat open and disconnect the fan so it can't chop your hands off. Now with the fan disconnected have a feel over the entire area of the rad - you will feel any blockages as cold spots.

Just one other thought, rad caps are a not uncommon failure on the cars. Have you tried replacing it? They're less than a tenner.

Phil.

[mongoose]

PJLarge said:

HGF is highly unusual on an MX5, as are cracked heads - it's an alloy head against an iron block. Your best bet will be to get the car up to temp on the drive, let the t-stat open and disconnect the fan so it can't chop your hands off. Now with the fan disconnected have a feel over the entire area of the rad - you will feel any blockages as cold spots. Just one other thought, rad caps are a not uncommon failure on the cars. Have you tried replacing it? They're less than a tenner. Phil.

Thanks for that Phil.My garage have tested for hgf and its all clear,so looks as though your spot on with the rad or cap.My limited knowlege says it wont be the cap,as the overflow is immediately beneath it in the filler neck,and this is where the pressured air is going.Its not as if the cap is passing,which to me is the only way it could fail?The rads being tested on Monday,so we'll see then.Many thanks for your advice,really appreciate it,cheers,Mike